A pair of Zika vaccines, given to mice before pregnancy, protected their offspring from Zika infection. While they have not been tested in mice that are already pregnant, the study is a step toward tangible protection for pregnant women, who risk fetal brain defects if they are exposed to the virus.
There are multiple Zika vaccines in the works, including those from Inovio Pharma, Themis and Sanofi. However, none are designed to be given during pregnancy. Researchers from Washington University in St. Louis, the University of Texas Medical Branch and the National Institute of Allergy and Infectious Diseases tested the ability of two in-development vaccines to protect mouse fetuses whose mothers were infected while pregnant.
The first vaccine, a “subunit” injection from Moderna Therapeutics, is in safety trials in men and women who are not pregnant. The second, which was developed at the University of Texas and contains a live, weakened form of Zika, is in animal trials.
The team vaccinated groups of mice with one of the vaccines or a placebo. They were then infected with Zika in early pregnancy.
A week after infection, the researchers found that, among the mice treated with the subunit vaccine, more than half had no Zika RNA at all in their placenta and fetus, according to a press release. As for those given the live vaccine, 78% of placentas and 83% of fetuses showed no trace of viral RNA. The vaccinated mice birthed pups with no detectable Zika virus in their heads. The findings are published in the journal Cell.
The unvaccinated mice carried much higher levels of Zika material. None of them completed the pregnancy—they became seriously ill, their placentas were damaged and many fetuses died in utero.
Because mouse pregnancies are so short, the study did not gauge the vaccines’ efficacy when given during pregnancy, nor did it address the vaccines' utility in sexually transmitted Zika.
"The question is, 'Would the immunity still hold up if the virus does not pass through the bloodstream?'" said Michael Diamond, M.D., Ph.D., professor of medicine at Washington University, in the release. "We think it will hold up, but that has to be tested."
Zika typically causes mild symptoms, or no symptoms at all, unless it infects a pregnant woman. While the CDC has definitively linked Zika infection to microcephaly, its current recommendations for pregnant women range from avoiding travel to high-risk areas, practicing safe sex and taking precautions to avoid mosquito bites. A vaccine or a drug for pregnant women would be a game changer. Another team from Washington University is working on repurposing a broadly used malaria drug to block Zika transmission from mother to fetus.
"In general, most doctors don't want to vaccinate during pregnancy on the outside chance that the immune response itself could harm the fetus," Diamond said. "But if you're in an area where Zika is circulating, you might vaccinate during pregnancy because the risk of Zika infection is worse than some theoretical risk of immune-mediated damage."